/*
 * Kodkod -- Copyright (c) 2005-present, Emina Torlak
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
package kodkod.engine.ucore;

import kodkod.engine.fol2sat.TranslationLog;
import kodkod.engine.fol2sat.Translator;
import kodkod.engine.satlab.ReductionStrategy;
import kodkod.engine.satlab.ResolutionTrace;
import kodkod.util.ints.IntCollection;
import kodkod.util.ints.IntIterator;
import kodkod.util.ints.IntSet;
import kodkod.util.ints.Ints;

/**
 * Simple Core Extraction is a strategy for generating unsat cores that are
 * minimal at the logic level. Specifically, let C be a core that is minimal
 * according to this strategy, and let F(C) be the top-level logic constraints
 * corresponding to C. Then, this strategy guarantees that there is no clause c
 * in C such that F(C - c) is a strict subset of F(C). Furthermore, it also
 * guarantees that for all f in F(C), F(C) - f is satisfiable. This is a
 * stronger guarantee than that of {@linkplain HybridStrategy}. In general,
 * using this strategy is more expensive, timewise, than using
 * {@linkplain HybridStrategy}.
 * <p>
 * This implementation of SCE will work properly only on CNFs generated by the
 * kodkod {@linkplain Translator}.
 * </p>
 *
 * @author Emina Torlak
 * @see HybridStrategy
 */
public final class SCEStrategy implements ReductionStrategy {

    private final IntCollection varsToTry;

    /**
     * Constructs an SCE strategy that will use the given translation log to relate
     * the cnf clauses back to the logic constraints from which they were generated.
     */
    public SCEStrategy(final TranslationLog log) {
        varsToTry = StrategyUtils.rootVars(log);
    }

    /**
     * {@inheritDoc}
     *
     * @see kodkod.engine.satlab.ReductionStrategy#next(kodkod.engine.satlab.ResolutionTrace)
     */
    @Override
    public IntSet next(ResolutionTrace trace) {
        if (varsToTry.isEmpty())
            return Ints.EMPTY_SET; // tried everything
        final IntSet relevantVars = StrategyUtils.coreTailUnits(trace);

        for (IntIterator varItr = varsToTry.iterator(); varItr.hasNext();) {
            final int var = varItr.next();
            varItr.remove();
            if (relevantVars.remove(var)) { // remove maxVar from the set of
                                           // relevant variables
                if (relevantVars.isEmpty())
                    break; // there was only root formula left
                // get all axioms corresponding to the clauses that
                // form the translations of formulas identified by relevant vars
                final IntSet relevantClauses = StrategyUtils.clausesFor(trace, relevantVars);
                assert !relevantClauses.isEmpty() && !relevantClauses.contains(trace.size() - 1);
                return relevantClauses;
            }
        }
        varsToTry.clear();
        return Ints.EMPTY_SET;
    }

}
